The invention relates to a hybrid propulsion system for a vehicle provided with combined mechanical and hydraulic propulsion. The invention is in particular directed to a cooling system for such a vehicle.
For heavy road vehicles, it is known that there is sometimes a desire for providing driving force on several wheel pairs such that the vehicle for example is provided with a driving force on a rear pair of wheels as well as on front pair of wheels. In many cases, it is desirable to be able to control the traction of the vehicle such that one or several wheel pairs may be connected or disconnected from the power source depending on the traction force demand. The propulsion unit may be the same for all the driving wheels or be a combination of different power sources, e.g. a vehicle provided with a mechanical drive train connected to an internal combustion engine and a hydraulic power source connected to hydraulic motors. In general, the mechanical drive is used as the main propulsion system for road travel and the hydraulic drive used as an auxiliary drive for rough conditions at low speeds or for a vehicle in a work mode, e.g. when using creep drive for loading or unloading operations. Different examples of such vehicles are for example disclosed in WO 2011/100 206; U.S. Pat. Nos. 5,361,208; 3,780,820; EP 505 727; or US 2011/197 574.
Vehicles today are thus provided with traction forces on both rear wheels and front wheels using a combined mechanical and hydraulic drive and are designed and have control systems for enabling, disabling and controlling the different traction systems to be used efficiently. In order to provide an efficient traction using a combined mechanical and hydraulic system it is important to be able to manage the cooling of vehicle components including the hydraulic propulsion system. In order to provide an efficient cooling system it is generally considered that the cooling system used for the hydraulic propulsion system also may be used for other cooling demands in the vehicle. In U.S. Pat. No. 7,261,068 is described a cooling system for a work vehicle which may be used for cooling hydraulic oil as well as the engine, transmission oil and charge air.
However, even though cooling systems are known which are designed to manage essentially all cooling desired for a vehicle, as for example in U.S. Pat. No. 7,261,068, there is still a demand to develop a cooling system which is designed for the cooling of a heavy road vehicle provided with a hybrid propulsion system comprising mechanical and hydraulic propulsion systems in order to optimize the cooling and improve performance of the vehicle.
It is desirable to provide an efficient cooling for a hydraulic propulsion system which may be used for cooling hydraulic liquid for the propulsion system as well as other parts of the vehicle. The cooling system is designed for a hydraulic propulsion system used as an additional propulsion system together with a mechanical propulsion system to power wheels of the vehicle.
In general, the hydraulic propulsion system is intended to only be active during slow speeds, for example when there is a desire for additional power or used as the single traction source when performing work or driving slowly with frequent stop and go. It is therefore a desire to be able to design the cooling system such that it may cool the hydraulic propulsion system and other parts of the vehicle in dependence of the total cooling demand.
Since the hydraulic drive is not intended to be used at certain conditions, e.g. when using the vehicle at rather high speeds, the cooling demand for the hydraulic propulsion system differs from the cooling demand for the mechanical propulsion system. Hence, there are peculiar design issues when designing a cooling system for a hybrid heavy road vehicle which causes the cooling system to include certain features in order to function in an efficient way.
The present invention is thus directed to a cooling system for a heavy road vehicle comprising a hybrid propulsion system including a first, mechanical propulsion system and a second, hydraulic propulsion system. The first, mechanical propulsion system comprises a mechanical drive train including an internal combustion engine which provides a traction force to a first traction wheel via a gearbox. The second, hydraulic propulsion system comprises a hydraulic pump unit for powering a hydraulic motor in order to provide a traction force to a second traction wheel.
The cooling system further comprises a pump and a first cooling circuit including a gearbox cooling circuit, a hydraulic propulsion cooling circuit and a first radiator for cooling of a coolant flowing in the first cooling circuit. The radiator is connected in series with and located upstream of the gearbox cooling circuit and the hydraulic propulsion cooling circuit.
The cooling system of the hybrid heavy vehicle is designed such that the hydraulic propulsion cooling circuit and the gearbox cooling circuit are connected in parallel.
The cooling system further comprises a temperature dependent thermostat which is positioned downstream the hydraulic cooling circuit, the temperature dependent thermostat is configured to increase the coolant flow through the gearbox cooling circuit relative the coolant flow through the hydraulic propulsion coolant circuit if a measured temperature of the coolant flow downstream the gearbox cooling circuit exceeds a predetermined threshold temperature.
The measured temperature may be measured by means of a temperature measuring device, such as e.g. a thermometer or a temperature sensor. The temperature measuring device may be configured to measure the temperature of the coolant flow downstream of the gearbox cooling circuit. The predetermined temperature threshold may vary depending on the circumstances and specific use of the vehicle the system is provided to. The skilled person is familiar with gearbox temperatures that may adversely affect the gearbox both in a short time perspective as well as in a long time perspective.
According to a further advantage of the present invention, a smaller cooler, in terms of cooling power, can be used for the system of the present invention, since the cooler does not have to be able to supply cooling for a required maximum cooling of the gearbox and the hydraulic system at the same time.
The cooling system may further comprise a second cooling circuit which comprises an engine cooling circuit. This second cooling circuit is arranged in parallel with the first cooling circuit. By arranging the cooling circuits in parallel it is possible to provide coolant flows having different properties, e.g. temperature and mass flow, in order to achieve a desired cooling effect for different purposes. In general, there is a desire to provide a coolant having a lower temperature for the hydraulic liquid in the hydraulic propulsion system and the gear box lubrication oil than for the engine cooling circuit. By providing two parallel coolant flows it is thus possible to get the desired lower coolant temperature for the first circuit without the need to use coolant at a too low temperature for the second circuit and thereby achieve a desired cooling of the engine without wasting energy in reducing the coolant temperature to an unnecessary cold temperature. In addition, providing two separate flows at different temperatures may also make it easier to cool further parts of the vehicle by using a coolant being in a suitable temperature range or allow a more efficient heating of certain parts as will be discussed further below.
The cooling system may further be designed such that a second radiator is located in the cooling system and arranged to receive a return flow from the second cooling circuit. According to one embodiment, this second radiator may be bypassed by the return flow from the first cooling circuit. The cooling system may thus be arranged such that the coolant flowing in the first coolant flow bypasses the second radiator but is returned to the cooling system upstream a pump while the flow from the second cooling circuit is directed through the second radiator before it is mixed with the coolant flow from the first coolant circuit upstream of the aforementioned pump such that both the flow from the first and second cooling circuits are mixed upstream a common pump for both return flows. Hence, the return flow from the second cooling circuit and the return flow from the first cooling unit may thus be used as inflows to a pump common for both circuits. The flow from the pump is thereafter divided to a flow entering the second circuit and a flow entering the first circuit which is cooled by the first radiator before it will flow on to either of the parallel connected hydraulic propulsion coolant circuit and gear box cooling circuit.
In a still possible configuration of the coolant system, the return flow from the first cooling circuit may also be directed to pass through the second radiator, either the complete flow or having a divided flow in which a part of the return flow is directed to pass through the second radiator and a part of the flow is bypassing the second radiator. The partial flows may be controlled by a valve.
It is also possible to design the return flow of the second cooling circuit such that it comprises a bypass conduit which bypasses the second radiator. The proportion of coolant flow in the second cooling circuit which passes through the second radiator respectively the bypass conduit may be controlled by a thermostat. The thermostat may be arranged in the second cooling circuit at a position downstream the engine cooling circuit, such that a return flow from the second cooling circuit is configured to be controlled by the thermostat to control the proportion of the coolant flow which passes through the second radiator or a bypass conduit bypassing the second radiator. The bypass flow may be used in particular at start up conditions when the coolant is rather cold and there may be a desire to use the coolant in the second cooling circuit for heating purposes as will be discussed further below. Since the second cooling circuit includes the engine cooling circuit is the coolant flow in this circuit normally heated quicker than the coolant flow in the first cooling circuit why the coolant in the second cooling flow in general is more suitable to be used for heating.
The first radiator and the second radiator are preferably air to coolant heat exchangers in which a fan is used to produce an air flow through the heat exchangers in order to cool the coolant. The radiators may be arranged in close proximity to each other and be cooled at least partially by the same air flow. Hence, the first and the second radiator may be arranged to be cooled by means of a common air flow fan. By such an arrangement only one fan may be needed. Since there in general is a desire for a cooler coolant in the first cooling circuit it is usually an advantage to locate the first radiator upstream of the second radiator in said air flow such that the coldest air is used in the first radiator.
Concerning the control of the coolant flows which flows through the hydraulic propulsion cooling circuit and the gearbox cooling circuit in the first cooling circuit may the proportion of the parallel flows be divided in dependence on the temperature of the gearbox. The cooling of the gear box is prioritized compared to the cooling of the hydraulic propulsion system since the hydraulic propulsion system may be turned off if overheated while the gearbox is essential during almost all driving conditions. Hence, the flow in the first circuit is controlled such that the flow of coolant through the gearbox cooling circuit is increased relative the flow through the hydraulic propulsion cooling circuit if the measured temperature of the gearbox is increased, i.e. exceeds the predetermined temperature threshold as described above. The temperature dependent control of the relative flow through the hydraulic propulsion cooling circuit and the gearbox cooling circuit may be controlled in dependence of the temperature by means of the thermostat. Again, the thermostat may be set to be sensitive to the temperature of the return flow of the coolant from the gear box cooling circuit. Above a certain temperature limit the thermostat may automatically change the relative flow through the hydraulic propulsion cooling circuit and the gearbox cooling circuit such that the flow through the hydraulic propulsion cooling system is reduced and thus the flow through the gearbox cooling unit is increased. The temperature dependent thermostat may be a temperature sensitive mechanical wax valve. Other types of temperature dependent thermostats are of course conceivable, such as e.g. a three-way valve, a check valve, etc.
Under certain conditions it may be desired to reduce the cooling of the hydraulic propulsion system, e.g. in cold conditions while the coolant and the hydraulic liquid is substantially cold by the environment. In this case the flow of hydraulic liquid through the heat exchanger in the hydraulic propulsion cooling circuit may be reduced. This may be achieved by incorporating a bypass conduit for the hydraulic liquid circuit in the hydraulic propulsion cooling circuit which allows a flow of hydraulic liquid to bypass the heat exchanger in which heat is exchanged between the coolant and the hydraulic liquid. The flow through the bypass conduit may be configured to be controlled in dependence of a measured temperature in the hydraulic propulsion cooling circuit and/or the coolant temperature, e.g. by means of a temperature regulated valve in the bypass conduit which is closed when the temperature is indicated to be sufficiently high but closed in response to a temperature indication below a certain value. The valve could be electronically controlled or be designed to mechanically open up automatically when subjected to a coolant temperature below a certain limit.
The cooling system may further include the feature of including one or several heating circuits in the second cooling circuit. By a heating circuit is meant a circuit configured for heating purposes, e.g. a urea heating circuit and/or a cab heating circuit. These heating circuits are preferably connected in parallel with the engine cooling circuit, alternatively arranged in series with the engine cooling circuit and provided with bypass conduits such that the heating circuit may be heated or bypassed by changing a bypass valve. If the heating circuits are connected in parallel with the engine cooling circuit they could be arranged to have its inlet connected to the cooling system downstream the pump, before the engine cooling circuit and its outflow connected to the bypass conduit return flow in the second cooling circuit, i.e. the return flow conduit bypassing the second radiator. The heating circuits are advantageously located in the coolant flow receiving heat from the engine cooling circuit in order to provide a rapid heating.
The second cooling circuit may include further cooling circuits as well, e.g. a retarder cooling circuit may be connected in series with the engine cooling circuit, preferably downstream the engine cooling circuit.
The first cooling circuit may be designed such that the return flow is connected to the cooling system downstream of the second radiator.
The hydraulic propulsion cooling circuit is preferably a liquid to liquid heat exchanging arrangement and comprises a hydraulic liquid circuit which is in thermic contact with the first cooling circuit in a heat exchanger. In order to be able to reduce the cooling of the hydraulic propulsion system, the hydraulic liquid circuit may be provided with a bypass conduit, bypassing the heat exchanger in the hydraulic propulsion cooling circuit, comprising a valve. The valve may be configured to open in dependence of the temperature of the coolant and/or the temperature of the hydraulic liquid in the hydraulic propulsion cooling circuit, i.e. the valve opens up when the temperature is below a certain limit, either completely or to different degrees proportional to the temperature. The purpose is thus to avoid cooling of the hydraulic liquid when its temperature is below a desired working temperature.
According to an example embodiment, a control unit may be configured to turn off the hydraulic propulsion system is a measured temperature of the hydraulic propulsion coolant circuit and the gearbox cooling circuit is above a predetermined temperature threshold limit.
Hereby, if the cooling demand for the hydraulic system and the gearbox is larger than the cooling capability of the cooling system, the control unit can turn off the hydraulic propulsion system to only provide coolant to the gearbox. Accordingly, it is the total temperature for the hydraulic propulsion system and the gearbox system that is compared to the threshold limit.
The threshold limit can be set by the skilled person in such a way that temperatures below such limit are not adversely affecting either the hydraulic system as well as the gearbox. It should also be readily understood that the specific control unit can be any control unit already present on the vehicle. The control unit can hence also receive, from temperature sensors, the measured temperature of the hydraulic propulsion cooling circuit and the gearbox cooling circuit.
The invention also relates to a heavy road vehicle provided with a hybrid propulsion system comprising mechanical and hydraulic propulsion units provided with a cooling system as defined above.
Concerning the cooling system described above, it is obvious that it may comprise further valves, sensors, pumps cooling circuits or other features commonly used in cooling systems. The system disclosed above describes certain features which advantageously may be used in a wide variety of systems.